To produce rebonded foam, polyurethane flexible foam shredded to flake size is mixed with a binder, for example a polyurethane prepolymer, introduced into a box mould and compressed to the desired density or block height. The free isocyanate groups still present in the polyurethane prepolymer react, for example, with moisture from the environment and form urea compounds. To accelerate the reaction, saturated steam may also be conveyed through the base of the box mould and through the foam block. This measure may considerably reduce the residence time or “mould dwell time” of the foam blocks in the box mould.
The box moulds used conventionally comprise a round or rectangular cross-section, which is constant over the height of the box mould.
When introducing the foam flakes into the box mould, the foam flakes are conventionally distributed manually or by a suitable device evenly over the cross-section of the box mould, in order to prevent differences in density in the horizontal direction in the finished foam block.
Because of the inherent weight of the foam flakes and the low compressive strength of the bed of foam flakes, however, there is a considerable difference in density in the bed in the vertical direction. Therefore, the bed has a markedly higher density in the lower area than in the upper area.
The result of this is that the finished foam block obtained after compression has a higher density in the lower portion than in the upper portion. This undesired effect is more noticeable the taller the box mould and thus the foam block is and the lower the density of the finished foam block is.